1. Field of the Invention
The invention pertains to the field of cryorefrigeration. More particularly, the invention pertains to a multi-stage pulse tube cryocooler.
2. Description of Related Art
Typical closed-cycle regenerative cryocoolers include the Stirling, Gifford-McMahon and pulse tube types, all of which provide cooling through the alternating compression and expansion of a working fluid, with a consequent reduction of its temperature. Stirling and Gifford-McMahon regenerative cryocoolers use displacers to move a working fluid (usually helium) through their regenerators. The noise and vibration induced by the displacer creates problems, and the wear of the seals on the displacer require periodic maintenance and replacement.
Therefore, it is highly desirable to invent cryorefrigeration devices that generate less vibration and less acoustic noise than prior art cryocoolers. It is also desirable to decrease the number of moving parts used in cryorefrigeration devices and to significantly increase the required maintenance intervals and reliability. Pulse tube cryocoolers are a known alternative to the Stirling and Gifford-McMahon types, which differ from these in that pulse-tube cryocoolers do not use a mechanical displacer.
A pulse tube is essentially an adiabatic space wherein the temperature of the working fluid is stratified, such that one end of the tube is warmer than the other. A pulse tube cryocooler operates by cyclically compressing and expanding a working fluid in conjunction with its movement through heat exchangers. Heat is removed from the system upon the expansion of the working fluid in the gas phase.
As used herein, a “stage” in a cryocooler is a location in the cooler at which gas expansion and refrigeration occurs, and at which a thermal load may be attached at a “cooling station”.
Prior art single-stage valved pulse tube cryocoolers generally include a pulse tube, a rotary valve to generate the oscillating compression-expansion cycle, a reservoir to contain the expanding working fluid gas, orifices for the movement and phasing of the gas between the reservoir or buffer volume and the rest of the system, and a regenerator for absorbing heat temporarily and reversibly. Single stage pulse tube cryocoolers are generally capable of reaching temperatures above 20K., and achieving lower temperatures has in the past required staging of the pulse tubes. U.S. Pat. No. 3,237,421 to Gifford and other prior art publications disclose multistage pulse tube cryocoolers. U.S. Pat. No. 5,295,355, a 1994 patent issued to Zhou, et al, shows a single-stage multi-bypass refrigerator.
Prior art two-stage pulse tube cryocoolers generally include, in addition to the foregoing components, a first-stage pulse tube, a first-stage regenerator, a second-stage pulse tube, a second-stage regenerator and first and second cooling stages. FIG. 2 of U.S. Pat. No. 6,378,312, issued to the present inventor, shows a two-stage cryocooler.
US Published application 2003/0163996 shows a cryocooler having two pulse tubes 108 and 120 and regenerator 106. A “heat intercept” 202 (FIG. 2) connects second stage pulse tube 120 and regenerator 106, but there is no flow passage in this connector. Because there is no gas passage, no gas expansion or refrigeration occurs at this point, so this invention is not a true “three stage” cooler, as the term is defined and used herein.
In a 1997 paper in Cryogenics (vol. 37, No. 12, pp. 857-863), entitled “Experimental study of staging method for two-stage pulse tube refrigerators for liquid 4He temperatures”, the present inventor, plus Thummes and Heiden, described several embodiments of two-stage cryocoolers. The cooler shown in figure (c) is an attempt by the inventor to increase efficiency in the second stage by adding a gas bypass orifice between the second stage pulse tube and the second stage regenerator to control phasing of gas flow within the second stage. There is no heat exchanger at the location of the bypass, so that there is no cooling station located at this bypass, therefore this is a two-stage, not a three-stage cooler.
Three-stage cryocoolers are also known in the prior art which include all of the parts of a two-stage pulse tube cryocooler, plus a third reservoir, a third-stage regenerator, and a separate third-stage pulse tube in parallel with the first- and second-stage tubes.